Digital compensation of Chromatic Dispersion (CD—the dependency of the phase velocity of an optical signal on its wavelength) and Polarization Mode Dispersion (PMD—modal dispersion where two different polarizations of light in a waveguide, propagate at different speeds, causing random spreading of the optical pulses) in 40 Gbp/s and 100 Gbp/s coherent optical fiber communication systems is of great interest nowadays. Today, the possibility to accurately detect not only the amplitude but also the phase of data carrying optical signals allows transmitting data by using the phase of these signals. Therefore, coherent detection and equalization allows compensating distortions that are introduced by very long fibers.
Today, sampling of optical signals in rates of 100 Gb/s is problematic, since even though the information is carried by 28 Gb/s using different polarization and splitting the data into I and Q (In-phase and Quadrature) channels, the sampling rate must be 56 Gs/s. This requires special equipment with resolution of about 5 bits, which is expensive. In addition, there is a demand for transmitting and receiving in rages of 400 Gb/s, which pushes the limits of feasible equipment even much further.
The common practice of CD and PMD compensation is to use fractional space equalizers, with two samples per symbol, or even more. It is well known that in undistorted media, sampling at the symbol rate forms sufficient information to recover the digital data. However, when the channel introduces linear distortions such as CD and PMD, a full reconstruction of the received analog signal is required in order to apply digital compensation. Sampling this signal at the symbol rate without preceding filtering violates the Nyquist sampling theorem, causing aliasing effect that results in performance degradation. On the other hand, using Anti Aliasing Filtering (AAF) prior to symbol rate sampling introduces substantial low-pass filtering which, in turn, causes substantial Inter Symbol Interference (ISI). The optimal equalizer, in the sense of minimum probability of error for a channel with ISI is the Maximum Likelihood Sequence Estimator (MLSE).
Several attempts of dealing with symbol space equalizers were made using AAF, in order to reduce cost and complexity of VLSI implementation. However, these attempts deal only with low CD values suffer from significant power penalty due to the combined effects of Aliasing and ISI.
All the methods described above have not yet provided satisfactory solutions to the problem of optimally equalizing the distortion of an optical data channel, while using reduced sampling and processing rates.
It is therefore an object of the present invention to provide a method and system for optimally equalizing the distortion of an optical data channel, while using reduced sampling and processing rates.
Another object of the present invention is to provide a method and system for equalizing the distortion of an optical data channel, without the need for expensive sampling and processing equipment.
It is a further object of the present invention to provide a method and system for equalizing the distortion of an optical data channel, while introducing low power penalties.
Other objects and advantages of the invention will become apparent as the description proceeds.